I've just reviewed most of the issues other players have with this game. And, it looks like, IG can answer all of them at once.
People don't like that there is no cover system, though, yes, there is one, but when it comes to DoW3 it's more about "the feel" than of anything else. So, yep, why don't you give them the old buildables with ranged defenceive aura-projecting? There are pieces of scrap already, which can be used in exact same way. It should be 100% good, if IG won't have directly-applied knockback\melee units up to the later tiers. After all, it's what CoH fans were about past days, I mean for CoH2 to have melee values on units. Oh, if it's concerning you that buildables can be turn into the road-blocks on narrower maps, well, they can be limited by the proximity towards each other (if that would be ever need). DoW3 factions already have any means necessary to deal with such covers, such as flamers, grenades and melee units, but that can be extended with the need to "attach" your squads to cover limited by the time, to make knockback more effective.
2nd, there is no supression? If people don't like the slow, well, you can rename it into supression with some stacking mechanism like Nobz'es and Chapellain's "Fury" divided on phases. I'm well aware that supression in CoH series goes toe-to-toe with "withdraw" capability, but, ehmm, you could always cut it effects like reduction in speed or damage reduction and others, the thing Relic pull with DoW2.
Speaking of withdrawals.. this could be added, too. It should be Ok unless it limited with cooldown and core infatry's HP would be on such a level when most of the encounters would bring them casuality anyway. I've said "core infantry"? Right, another curtsy to CoH. The basic infantry rooster may be just the one of CoH, namely: core infantry, MG, a mortar (grenade launcher team or whatever), a sniper. And by "sniper" I don't mean that sniper hero from DoW1, but some "tee-hee" sniper and scout in one person that could miss it's target based on number of models in the squad (as it should be one of it's kind in the game, I mean it as a balance measure to save SM's from him). So, there is a basic RNG already, so why not add things like "far" and "close" damage multiplayer for the core infantry (it's "accuracy" in CoH and DoW2, but who cares?), since there is no melee fighters on T1 for IG; something that spread damage over the models close to each other (like MG42, that does a lot of damage, but not kills) and, well, a mortar. I suggest, it's already in the engine and wraith-units recall is nothing but withdraw, isn't it?
One more thing people were complaining that there is no squad upgrades and commanders in the game. Well, with a single massed core infantry unit, I guess, it's quite a trivia to achieve in terms of balance. After all, it is what IG was in DoW1 prior DC. Elites can be made attachable to common squads, passing them some kind of effect (yep, that should make them totally generic, and that relates to "MOBA-like" acqusations and "Elites are winning the game, base units are useless". This time, elites can be made useless without asignement to a squad carrying some sufficient bonus for their EP value).
Oh, and to savour things, since I've thought that up, core infantry could make a stackable on ally's death scale, like the very "Fury", forcing them to retreat or receive a de-buff when it's full. I guess, it passes as "morale".
To sum up things, I don't think such a faction would drastically affect those ones already present. DoW3 is a good game and could be expanded in all directions, like introducing veterancy, easy-scripting mode like those of Ashes of Singularity (only the basic and limited part of it, but still) and many other things for many new factions, but this exact expansion could bring the largest, to my mind, amount of players from DoW1, DoW2 and CoH, as well as being the most "Relic-like" and providing the greatest "feel".